This application claims priority under 35 U.S.C. §119(a) on Patent Application No. 2007-325092 filed in Japan on Dec. 17, 2007, the entire contents of which are hereby incorporated by reference.
The present invention relates to an intermediate transfer-type image forming apparatus provided with a transfer belt that primarily transfers a toner image formed on a photosensitive drum, a secondary transfer unit having a transfer roller that secondarily transfers the primarily transferred toner image to a paper, and an optical sensor that detects a reference toner image on the transfer belt.
Recently, electrophotographic image forming apparatuses that are capable of forming a multicolor image, for example, a color copier and a color printer, have been developed. For example, in a well-known intermediate transfer-type image forming apparatus, an image is formed by forming a toner image for each color on a latent image carrier such as a photosensitive drum; forming a multicolor image by transferring the toner image for each color one-by-one in overlaying fashion to a transfer belt, i.e., an intermediate transfer body; and transferring the multicolor image to a recording paper, i.e., a transfer paper, and then fixing the multicolor image to the recording paper.
An example of such an image forming apparatus includes an image forming apparatus including a transfer belt that primarily transfers a toner image formed on a photosensitive drum, a secondary transfer unit having a transfer roller that secondarily transfers the primarily transferred toner image to a paper, an optical sensor that detects a reference toner image on the transfer belt, and a shutter that protects a detection face of the optical sensor, and structured so that the secondary transfer unit and the shutter are coordinated (for example, see JP H7-234595A).
Although the image forming apparatus described in JP H7-234595A is not an intermediate transfer-type, as shown in FIG. 9, the apparatus is structured so that a shutter 517 and a transfer roller (corresponding to the secondary transfer unit) 505 are coordinated by a disjunction means. That is, before a reference toner image 506 on a photosensitive drum 501 reaches a position at which the reference toner image 506 is brought into contact with the transfer roller 505, a solenoid 512 is energized. In this way, an actuator 512a is pulled in by the solenoid 512, and along with that, an arm member 509 rotates in the clockwise direction around a spindle 510, the transfer roller 505 attached to one end of the arm member 509 moves from the position where it is in contact with the surface of the photosensitive drum 501 to a position where it is separated from that surface, indicated by reference numeral 505′. Also, along with the rotation of the arm member 509, a shutter arm member 516 that is linked to the arm member 509 also rotates in the clockwise direction around the spindle 510, and the shutter 517 moves along a guide member in the direction of the arrow, from a protection position to an evacuated position, evacuating so that a reflective optical sensor 507 can detect optical density of a reference toner image 506′.
Then, after the reference toner image on the photosensitive drum 501 has passed through the position where it faces the transfer roller 505 and the reflective optical sensor 507 has detected the optical density, the solenoid 512 is de-energized. In this way, the arm member 509 and the shutter arm member 516 rotate in the counter-clockwise direction around the spindle 510 due to biasing force of a spring 511; the transfer roller 505 is brought into contact with the surface of the photosensitive drum 501 with a predetermined pressure, to be standing by for a next transfer operation; and the shutter 517 moves to the protection position. In FIG. 9, reference numeral 541 indicates a developer sleeve, and 545 indicates a developer bias power source.
As described above, in conventional image forming apparatuses, the shutter 517 that opens and closes so as to protect a detection face of the reflective optical sensor 507 is provided, and at the time of the density measurement, this shutter 517 is opened, and it is necessary that the transfer roller (secondary transfer unit in the case of intermediate transfer-type) 505 for transferring a toner image on the photosensitive drum (transfer belt in the case of intermediate transfer-type) 501 to a paper is separated from the photosensitive drum (transfer belt in the case of intermediate transfer-type) 501.
Furthermore, when such a shutter 517 is provided, it is conceivable that a controlling means controls a driving current using reflected light between facing planes of the shutter 517 and the reflective optical sensor 507 when the shutter is closed as a reference reflected light, in order to adjust the driving current of the reflective optical sensor 507 to adjust the irradiation light amount to a constant amount. Therefore, it is necessary that the shutter 517 is provided in proximity to the photosensitive drum (transfer belt in the case of intermediate transfer-type) 501. This is because, since outgoing light from the reflective optical sensor 507 is applied to the surface of the photosensitive drum (transfer belt in the case of intermediate transfer-type) 501, and its reflected light is detected by the detection face of a photo transistor, when the surface of the photosensitive drum (transfer belt in the case of intermediate transfer-type) 501 and the shutter 517 are not close to each other, there is a possibility that the reference reflected light from the shutter 517 will not reach the photo transistor.
As described above, it is necessary that the shutter 517 is disposed near the photosensitive drum (transfer belt in the case of intermediate transfer-type) 501. Therefore, in intermediate transfer-type image forming apparatuses, when attaching or removing the transfer belt, there is a risk that the shutter is brought in contact with the surface of the transfer belt, damaging the surface of the transfer belt. Thus, when attaching or removing the transfer belt to or from the main body of the apparatus, it is necessary to separate the shutter from the transfer belt, but no proposal has been made to configure conventional intermediate transfer-type image forming apparatuses so that the shutter is separated from the transfer belt when attaching or removing the transfer belt to or from the main body of the apparatus.